Recently, the scanning tunneling microscope (hereinafter abbreviated as STM) has been developed for observing the surface structure of a solid sample down to the atomlc level. In the STM, when a sharp pointed probe is brought within a distance of about 1 nm from the sample surface and a predetermined blas voltage is applied between the sample and the probe, a tunneling current flows between them. The intensity of the tunneling current is very sensitive to the distance between the sample and the probe. The probe is scanned over the sample surface in parallel with the sample surface by being controlled up and down in a direction perpendicular to the sample surface for keeping the intensity of the tunnel current at a predetermined value. By doing this, the unevenness of the sample surface can be observed by the trail of the probe.
The lateral resolution of the STM depends on how small the region of the sample is from which the tunneling current can flow, and that is defined by the radius of curvature of the probe tip. Furthermore, in the case of scanning a sample surface having a deep groove, when the cone angle of the probe tip 21 is larger as shown in FIG. 2(a), the probe tip 21 cannot reach to the bottom of the groove, and the tunneling current-flows between the edges of the groove and the side face of the probe. Therefore, the trail 24 of the motion of the probe does not reflect the shape of the sample surface 23. On the other hand, when the cone angle of the probe tip 21' is much smaller as shown in FIG. 2(b), the probe tip 21' can reach to the bottom of the groove, and thereby the trail 24' of the motion of the probe tip 21' sufficiently reflects the shape of the sample surface 23. Accordingly, it is desirable that the probe for STM have a very small radius of curvature and a very small cone angle at the tip part thereof.
Conventionally, such a probe for STM is made by sharpening the tip part of a fine wire made of tungsten, platinum or the like by electrolytic polishing or mechanical polishing. And it is known that the probe has a tip of less than 50 nm in radius of curvature with a cone angle ranging from 10 to 20 degrees at the tip part thereof (Journal of Vacuum Science & Technology A8(4), 3558-3562, 1990).
It, however, is difficult to manufacture the probe having a tip of less than 50 nm in radius of curvature with less than 10 degrees in cone angle artificially with reproductivity. Therefore, one having a predetermined shape is selected among the mass-produced probes in actual case.
On the other hand, there is known in nature a needle crystal having a tip of less than 50 nm in radius of curvature with less than 10 degrees in cone angie. In the needle crystal, such a very sharp tip is formed in a manner to have the most stable state in each step of the grain growth of the crystal. Therefore, there is not a large difference between the shapes of the individuals of a needle crystal. Accordingly, it is desirable to use such needle crystals to provide the probes for STM with reproductivity.
The very problem of the needle crystal in use of the probe for STM has the dimensions of the needle crystal. It is, in general, necessary that the probe has a length of about 10 mm since it is to be mounted on a main frame. However, such a lengthy needle crystal having a very small radius of curvature at the tip part thereof does not exist naturally. The length of the needle crystal is about 100 .mu.m at most. Accordingly, when a needle crystal is used as a probe for STM, the needle crystal must be fixed on a tip of a fine wire of metal having a sufficient length. Since STM detects the tunneling current flowing between a sample and a probe, it is necessary to have good conductivity between the needle crystal and the fine wire of metal.
The simplest method for fixing the needle crystal to the fine wire of metal is using a conductive adhesive such as silver paste. At first, the silver paste is spread on the tip part of the fine wire of metal, and the needle crystal is pushed onto the silver paste. Thereby, the needle crystal is adhered on the tip part of the fine wire of metal. At this time, if the needle crystal is strongly pushed onto the fine wire of metal, the silver paste might be adhered at the tip part of the needle crystal, or the needle crystal might be broken. Therefore, it is necessary to push the needle crystal weakly onto the fine wire of metal. On the contrary, when the silver paste is spread on the tip of the fine wire of metal, the surface of the silver paste becomes dry soon (for example within one minute), and a thin film forms on the surface of the silver paste, so that the adhesion of the needle crystal becomes difficult. Therefore, it is necessary to contact the needle crystal on the silver past as quickly as possible after spreading the silver paste at the tip part of the fine wire of metal. The probe configured above is dried an entire day and night, and after that the probe is generally used for observing the sample by STM. The needle crystal, however, is not compatible with the silver paste, that is the contact resistance between the needle crystal and the silver paste is large and the tunneling current can not flow between them stably. Furthermore, since the adhered area of the needle crystal and the fine wire of metal is small, even when the needle crystal contacts the sample softly, the needle crystal can be disconnected from the fine wire of metal.